Chromium electroplating process and product thereof

ABSTRACT

A process for producing a duplex coating made up of a chromium containing oxide (0.5 to 2 mg./ft. 2) overlying a metallic chromium layer (1.5 to 20 mg./ft.2) which is adherent to the base material. After the stock is plated in a first stage, it is passed through an electrolyte containing 40-100 g./1.CrO3 plus sulfate and/or fluoride catalyst to effect a dissolution and control of the overlying oxide layer. It is subsequently passed to a third stage (which may be omitted) where it is cathodically treated at a current density of less than 500 a./ft.2 to effect further control of the oxide layer. To reduce costs and simplify the process, all three stages may be accomplished in the same electrolyte bath.

United States Patent Allen et al.

PROCESS AND PRODUCT THEREOF lnventors: William S. Allen, Baldwin Borough; Guy Serra, Franklin Township, Westmoreland County, both of Pa.

Assignee: United States Steel Corporation Filed: A July 6, 1970 Appl. No.: 52,775

Related US. Application Data Continuation-in-part of Ser. No. 680,313, Nov. 3, 1967, abandoned.

-U.S. Cl. ..204/35 N, 204/41, 204/42 Int. Cl. ..C23f 17/00 Field 01' Search ..204/35 R, 35 N, 41, 42, 56

References Cited UNITED STATES PATENTS 3,296,100 1] 1967 Yonezaki et al ..204/41 Uchida et al Feb. 15,1972

OTHER PUBLlCATlONS .7 Aussieenlr9n8 S e nst 5/25/67- 0 Primary ExaminerJohn H. Mack Assistant Examiner-W. 1. Solomon AttorneyArthur J. Greif ABSTRACT A process for producing a duplex coating made up of a chromium containing oxide (0.5 to 2 mg./ft. overlying a metallic chromium layer 1.5 to 20 mg./ft. which is adherent to the base material. After the stock is plated in a first stage, it is passed through an electrolyte containing 40-100 g./l.CrO plus sulfate and/or fluoride catalyst to effect a dissolution and control of the overlying oxide layer. it is subsequently passed to a third stage (which may be omitted) where it is cathodically treated at a current density of less than 500 a./ft. to effect further control of the oxide layer. To reduce costs and simplify the process, all three stages may be accomplished in the same electrolyte bath.

11 Claims, No Drawings CHROMIUM ELECTROPLATING PROCESS AND PRODUCT THEREOF Thisapplication is a continuation-in-part of Ser. No. 680,313 filed Nov. 3, 1967 and now abandoned.

BACKGROUND OF THE INVENTION Flat-rolled steel has long been produced with an overlying layer of tin, either applied by hot dipping or, more recently, by electrodeposition. The tin layer serves as a protective, corrosion-resistant coating of particular value when the thus-coated steel is used in the manufacture of food and beverage cans and other containers requiring a corrision-resistant surface. The tin layer also facilitates soldering of the side seam construction usually' employed in the manufacture of such articles. New welding and bonding techniques have recently permitted the elimination of the costly tin layer.

Untinned fiat rolled steel stock, when used for canmaking purposes, must be provided with a protective coating, as enamel, lacquer and the like, to prevent deleterious chemical reaction between corrosive can contents and the metal can body. Such protective coatings are mostly frequently of a variety of compositions, being sufiicient in nature and function to the intended application of the coated container, and are preferably applied to the flat metal container stock by the container manufacturer, rather than by the steel producer.

Since the uncoated flat-rolled steel is susceptible to rusting during extended periods of shipment and storage, and since rusting detracts from the desired clean, bright appearance of the steel and also deleteriously affects adherence of subsequently applied protective coatings, it is essential that rusting be prevented and the steel delivered to the container manufacturer in a suitable condition for application and retention of the necessary protective coatings even after considerable periods of storage under humid conditions. Further, since the latter coatings are generally transparent, any treatment of the steel by the steel producer must not detract from the desired bright metallic appearance of the finished fabricated article.

Additionally, it is necessary that the metal stock provided to the can manufacturer be of such a nature as to resist delamination of the subsequently applied enamel or lacquer coating during fabrication, and also that the stock resist undercutting of such overlying protective coating by corrosive action of the can contents at the sites of defects in the enamel or lacquer coating or breaks, cuts or other defects caused during container fabrication.

Still further, in the case of containers having an adhesively bonded lapped side seam, it is required by the container manufacturer that such bond have a minimum peel (separation) strength of 25 pounds per inch of scam width for steel stock of 55 to 60 pounds basis weight (about 6 to 7 mils thickness).

Ordinary, untreated, cleaned and oiled flat-rolled steel (black plate) does not satisfy all of the foregoing requirements.

Most recently, tin-free steels have been made available for container manufacture and which provide, on the flat-rolled steel surface, either a thin layer of electrolytically deposited metallic chromium (for example, Uchida et al. U.S. Pat. No. 3,113,845), a film of chromium-containing oxides (for example, Kitamura U.S. Pat. No. 2,998,361), or both (for example, Yonezaki et a1. U.S. Pat. No. 3,296,100).

However, electroplating processes are inherently costly and, for their successful use in production of stock for container manufacture,.it is necessary that such electrolytic treatments add only insignificantly to the cost of production of untreated black plate. I

Specifically, such electrolytic processes, to be of maximum commercial utility and value, must meet the following economic criteria:

I. the cost of the electrolytic bath chemicals must be low;

2. the concentration of the bath chemicals must be low in order to avoid economic loss due to high dragout losses at' the required high line speeds, e.g.', 500 to 1000 feet per minute or greater;

3. the bath treating time must be low, in order to reduce to an economically feasible number, the treating tanks required;

. the power requirements must be low;

. the bath composition must be uncomplicated in preparation, control and maintenance;

6. the process must be sufficiently flexible in operation, so that only minor changes in process conditions can be made to produce a wide range of products of optimum characteristics for many end-use applications; and

7. the overall process must be susceptible of easy and accu rate controlto insure optimum product quality without excessive postproduction and prefabrication quality testmg.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a new and improved, low-cost process of electrodepositing thin but effective chromium layers on steel can stock and, in the same operation, to provide a protective and lacqueraherence-promoting hydrated chromium oxide film overlying the metallic chromium.

It is another object of the invention to provide a process for the control of the character of such hydrated chromium oxide films so that the required protective properties are uniformly and consistently obtained and undesirable colored films are avoided.

It is a further object to provide such processes which are readily and economically adapted to vary the weight (thickness) of the deposited'metallic chromium layer, in accordance with the requirements of different product end-use applications, while maintaining the necessary attributes of the overlying hydrated chromium oxide film.

It is yet another object of the invention to provide improved, low-cost products of a uniformly reproducible and desirable character, as produced by the inventive processes.

In accord with the above objects, the instant invention broadly comprises electrodepositing by methods well known in the art, a duplex coating made up of a metallic chromium layer and an overlying chromium oxide layer on a moving steel cathode, and thenin a second stage, treating the thus coated article with an oxide-solubilizing electrolyte, to control the character and thickness of the oxide layer. This solubilizing treatment may then be followed by a third stage, comprising the application of cathodic current to effect further control of the character and thickness of the oxide layer.

A preferred embodiment of the invention comprises employing the same oxide-solubilizing electrolyte for all three stages. The first stage comprising the application of a cathodic current density of at least 5O0a./ft. to provide a metallic layer having a thickness of 1.5 to 20 mg./ft. and an oxide thickness greater than 1 mg./ft. and then passing the cathode to the second stage, in which the oxide is controlled to a thickness of about 0.3 mgJft. to 2.0 mg./ft.". Further thickness control is then effected in the third stage by the application of a cathodic current below 500 a./ft.

The foregoing and other objects of the invention will be more readily understood by reference to the following description. v

DESCRIPTION The present invention provides an aqueous electrochemical bath and process by the use of which, in a simple, convenient, controllable, low-cost operation, there is provided a surfacetreated flat-rolled steel product, suitable for can manufacture, and having, deposited thereon both a metallic chromium layer and an overlying chromium-containing oxide film.

It was found, by static tests conducted with nonmoving steel strip, that when the steel is treated, as a cathode, in a chromiurn-plating electrolyte, e.g., in a low-cost, easily prepared aqueous solution containing 40 to 100 grams per liter (g./l.) of chromium trioxide (chromic acid), CrO together with a chromium deposition promoter and catalyst, such as sulfuric acid, e.g., in amount of 0.9 g./l. of sulfate ions, (S0,) under the influence ofa current of 200 to 500 a./ft. for 0.2 to 1 second, and if the strip is removed from the electrolyte substantially instantaneously after current termination, or while the passage of current is in progress, then the chromiumplated steel is covered by a gelatinous hydrated chromium oxide film of variable weight (thickness) and color, and which is also subject to ready streaking and mechanical disruption.

On the other hand, it was found that, if the steel is treated under the same conditions, but left in the electrolyte for a time upwardly of 0.5 second, e.g., to about 4 seconds, after the flow of current was terminated, the hydrated oxide film was not present, and the plated article exhibited a bright metallic chromium surface. However, further tests showed that the metal surface had poor rust resistance, poor adherence to the lacquers or enamels used in the container-manufacturing industry, so that such a product would require a further, postplating treatment in order to meet the property specifications of the can-making industry.

It was further discovered that, if the article were left in the electrolyte for a significant period of time, but not over about 0.5 second after current interruption, a bright article appearance is provided with improved rust resistance and enamel adhesion as compared to the article left longer in the current-free electrolyte.

[t was concluded that the electrolyte and the dissolution time had a pronounced chemical-solubilizing effect on the hydrated chromium oxide film deposited from the electrolyte under the influence of the plating current and that the thickness, color and resistance of the oxide film to mechanical abrasion might be controlled by a proper balancing of the opposed chemical and electrochemical processes.

It was also concluded, from the results of the above static tests, that it would be difficult or impossible to product bright chromium plated steel strip from such electrolytes in a continuous plating line of the type heretofore used in tinor chromium-plating, i.e., those wherein the steel strip is removed from the electrolyte while the plating current is flowing from a rotating conductor roll into the strip, and, further, that subsequent passage of the strip through downstream rolls would disrupt and smear the gellike oxide film.

These latter conclusions were confirmed in a series of experiments conducted on a moving steel strip, using a multiple pass plating arrangement, and the same type of electrolyte (50 g./l. of CrO as shown in Table 1, wherein Runs 680-1 to 6 were conducted with a sulfate ion concentration of 0.9 g./l. and Runs 680-14 to 18 with a sulfate ion concentration of 1.8 g./l. All runs-were conducted at a line speed of 200 feet per minute (f.p.m.).

TABLE 1 Current density, amps/ft but streaked It will be seen, particularly from the results of Runs 680-1 to 6, that, when there was no current flowing in the second pass, the sample was colorless, i.e., the samples exhibited the bright metallic appearance of chromium, whereas, when the current was left on the second pass, the samples took on a color ranging from amber to dark purple. These colors are attributable to heavy films of hydrated chromium oxide formed under the influence of the relatively high plating current densities.

On the other hand, in those samples where there was no current in the last pass, the samples, although bright, did have a discontinuous oxide deposit which was smeared and streaked, discontinuous oxide deposit which was smeared and streaked, giving an undesirable appearance.

These effects were less pronounced in the 680-14 to 19 series, due to the higher electrolyte acidity (1.8 g./l. $0 and the tendency of the more acid solution to dissolve the hydrated oxide.

In the preferred embodiment in which the same electrolyte is employed for all three stages, it is essential, for economic reasons, that the chromium-contributing material in the electrolyte be a low-cost one and be present in as small an amount as is compatible with good technical results. The electrolyte bath contemplated by the invention contains, as aforesaid, chromic acid as a readily available, low-cost, chromium-containing constituent, and is a relatively low concentration Forty g./l. of CrO has been found to be a minimum amount useful in the baths contemplated herein because below that amount bright chromium plate cannot be obtained by the process of the invention, and, further, a heavy, dark-colored coating of hydrated chromium oxide is produced.

The economics of the process become increasingly poorer as the concentration of CrO is increased above g./l. of electrolyte and, additionally, plating efficiency is lower at higher values and, still further, it becomes impossible to obtain sufficiently heavy initial hydrated chromium oxide films with more concentrated CrO solutions. The production of bright plate is an important criterion for selection of the catalyst used, since the amount and nature of catalyst affects hydrated chromium oxide film solution by the electrolyte.

After the first stage plating of the duplex coating has been effected, the character and thickness of the oxide layer may be controlled by the second stage dissolution process and subsequent third stage cathodic treatment at reduced current densities.

In general, for any given initial oxide thickness, the dissolution is a function of both the dissolution time and electrolyte composition. Shorter dissolution times being required as the amount of catalyst is increased, with fluoride being more effective than sulfate as an oxide film dissolver. Thus, when operating within the hereinafter described parameters of the invention, if one were limited to a configuration of baths and line speeds which made short dissolution times unobtainable or impractical, it would then be necessary to operate on the low side of the catalyst range. Since operating under such conditions would provide a slower" process, greater accuracy could be achieved in control of oxide thickness. Conversely, when operating under high line speeds and/or short length dissolving passes, one would employ catalysts on the high side of the catalyst range.

By employing such a dissolving stage, a duplex coating can be achieved with an oxide layer within the desired range of thickness. However, in many circumstances, it has been found desirable when operating high speed production lines, to effect an even greater degree of control on oxide thickness and further control the nature of the oxide layer by an additional cathodic treatment under reduced cathodic currents. Thus, if the oxide film exiting from stage 2 were found to be too thin or otherwise improtective, its protectivencss would be enhanced by the additional application of a cathodic current within the range of 0 to 500 a./ft. Similarly, when employing catalysts within the high side of the range, dissolution of most of the oxide layer will be accomplished in very short times (i.e., a

fraction of a second), thereby resulting in a diminution of process control. This diminished control can, however, be reacquired by employing the third stage cathodic treatment.

With an electrolyte containing from 40-100 g./l. of CrO an oxide layer plated at a speed of 500 ft./min. and a current density of 800 a./ft." can be dissolved to a thickness of less than 1.0 mg./ft. in a reasonably short time (about 4 seconds) with a catalyst containing as little as 0.2 g./l. sulfate. The process of this invention can, therefore, employ very small amounts of catalysts, since fluoride is even more effective as a catalyst than sulfate. While H 50. and HF have been employed in the foregoing experiments, other ionizable sulfate materials, i.e., those which, in water solution, provide the sulfate radical, 50 may be used, such as chromium sulfate, aromatic sulfonic acids and sulfonates, and other fluoride ion-prviding materials may be used, such as complex fluorides, as silicofluorides and the like. h WM it has been found that provision of the required properties for chromium-plated, flat-rolled steel stock intended for fabrication of containers, such as food and beverage cans and the like, is facilitated when the electrodeposited chromium layer is overlain by an adherent film of hydrated chromium oxide of a particular thickness range, i.e., thick enough to provide oxidationand corrosion-resistance and enhanced adherence of the lacquers and enamels used in can fabrication, but not so thick as to impart an undesirable color to the plated stock nor so thick as to impair lacquer and enamel adherence, or to result in mechanical film disruption.

it was further found that the magnitude of the current density applied to the steel article, as cathode, in the last pass of the article through the electrolyte is of critical importance in achieving oxide films of the desired character. Thus, a further series of moving strip tests are conducted, using an aqueous electrolyte containing 50 g./l. of CrO and 0.9 g./l. of concentrated sulfuric acid (together with various amounts of hydrofluoric acid), a line speed of 500 f.p.m., a first, plating pass current density of 700 a./ft. and variable current density in the second pass, with results as given in Table 2.

TABLE 2 Oxide film thickness, mg. Cr /ft.

0.6 ml./l. of 0.9 ml./1. of 1.4 ml./1. of 1.4 ml./1. of

After 15 minutes, the solution is removed to a beaker and heated to boiling to decompose excess peroxide. It is then cooled, neutralized to litmus with l to 3 H 80 and 5 ml. of sulfuric-phosphoric acid solution (200 ml. H 80 500 ml. distilled water, and 300 ml. of 85 percent H PO are added. Two ml. of 20 percent silver nitrate solution is added, then 5 ml. of fresh 30 percent ammonium persulfate solution are added, and one drop of 0.1 N potassium permanganate solution is then added. The solution is then heated to boiling for minutes after decomposition of excess persulfate. The solution is then cooled to room temperature, 3 ml. of 0.25 percent solution of diphenylcarbazide solution (0.25 gram diphenylcarbazide in 95 ml. of acetone plus 5 ml. of glacial acetic acid) are added and the solution diluted to 100 ml The absorbance of the solution is measured after 2 minutes at 560 mp. is a l-cm. cell in a Bausch and Lomb spectrophotometer or equivalent, calibrated against a standard curve prepared by similarly measuring the absorbance of a standard chromium solution. The latter solution contains 0.01 mg. Cr/ml. as prepared by dissolving K Cr O in distilled water.

It is desirable that a chromium-plating process of the type 'herein contemplated be susceptible to practice on existing tinplating lines with only minimal line modification. The process of this invention can be so used for the production of steel can stock having the needed property requirements. We have found that steel stock having a continuous, electrodeposited chromium layer ofa weight of about l .5 mg. per square foot of article surface (about 0.1 microinch thickness) will meet minimal property requirements for can fabrication purposes, and that chromium weights between about 3 to 4 rug/ft. will meet most rigid requirements. If the weight of the chromium layer is less than about 1.5 mg./ft. insufficient protection is afforded to the steel base to enable the article to meet the property specifications of the can-making industry.

Chromium layers of the foregoing minimum weights, and up to about 20 mg./ft. or more can be produced in accordance with the invention, with the use of from 3w 8 vertical plating H 8% 8% F, 48% HF, Current density in st Nos test Nos test Nos test Nos.

last pass, amps/ft. 8-12 13-17 19-23 2428 Appearance 0.6 0. 5 0. 8 0. 7 Colorless. 0. 8 0.7 0. 7 0.7 Do. 0. 8 0.7 0.7 0. 7 Do. 0. 9 0.8 0.8 0. 8 Slight darkening (amber). 0.9 1.1 1. 2 0. 9 Dark brown.

As will be seen from the Table 2 data, progressive increase in the last pass current density from 0 to 200 a./ft. resulted in an increase in the thickness of the hydrated chromium oxide film on the final article. Whereas the article remained bright, i.e., the film was colorless, up to a last pass current density of 75 a./ft. (oxide thickness of 0.7-0.8), kening of the article was obtained at current density of -100 a./ft. (oxide thickness of 0.8-0.9) and the sample became dark brown at a current density of 200 a./ft. (oxide thickness of 0.9-l .2).

Since the composition, and hence the density, of the hydrated chromium oxides formed on a steel article during chromium electrodeposition is unknown, andsince the very thin film thickness involved are not presently determinable by direct physical means, the thickness of the films can only be expressed indirectly. For the purposes of this disclosure and the appended claims, the amount of hydrated chromium oxide is determined and expressed at the amount of trivalent chromium content of the film per square foot of article surface. This determination is made as follows.

The oxide film on the metal surface is dissolved with a hot (about 90 C.) aqueous solution comprising 25 ml. of 1N sodium hydroxide and 2 ml. of percent hydrogen peroxide.

a slight darpasses of the type used in modern tin-plating lines, e.g., at line speeds of 1000 f.p.m. However, we have also found that little or no advantage is realized with chromium layer weights over about 10 mg./ft. to 12 mg./ft. (about 0.75 microinch thickness) as regards protection afforded to the steel base. Since heavier chromium layers add to the cost of the plated articles, we normally restrict the maximum chromium plate to weights of the latter magnitude, and preferably to about 6 to 8 mg./ft.

The percent plating efficiency is also an important economic factor in such chromium-plating processes, and the process of the invention provides excellent plating efficiencies, in the range of about 18 to 25 percent or higher. Best efficiencies have been found to be achieved when the electrolyte temperature is maintained within the range of about or C to about F, with somewhat better results being obtained when the temperature is kept on the low side of its range.

We have further discovered that the weight (mg. of Cr+++ /ft.") of the hydrated chromium oxide film is of critical importance as regards desired product properties, and that the weight range is a relatively narrow one, lying between about 0.5 mg. Cr-l-H-lft. on the low side, and about 2.0 and preferably 1.5 mg. Ci-+ll-/ft. on the high side. At weight values much below 0.5, the oxide film is of insufficient thickness to contribute the required rust-and corrosion-resistance or the necessary lacquer-adherence enchancement.

The following three examples of Table 3 are illustrated of the general principles of the inventive process. Treating passes prior to the last are termed plating passes and the last pass is termed the oxide" pass. This oxide pass is composed of two On the other hand, if the weight value is substantially over 5 zones. The first a dissolving zone with no applied current and about 1.5, the oxide film becomes colored and, moreover, the second consisting of the applying cathodic currents as inlacquer adherence is im air d, and th r du t i prone t dicated. In each of these three tests, the steel base was 60 mechanical smearing and streaking of the delicate oxide film Pound basis weight cold rolled. double-reduced low-Carbon during production and past-plating handling of the plates artiteel which had been given the preplating treatment described cle. However, if proper care is taken during handling and above. Current was applied to the strip through lead anodes. processing to avoid such mechanical smearing and streaking, TABLE 3 then uniform, protective films within the range of about 1.5-2.0 mg./ft. can be obtained.

Economical production of bright chromium-plated steel Process Parameter Example Example Example strip entails the use of high line speeds, e.g., at least 500 f.p.m. 2 3 and preferably 800 or 1000 f.p.m. or greater. It has been Chromic acid,g./l. so so 100 found that, at such line speeds, relatively high plating current cncemmed sulfur": d d d tl t 500 /ft 2 d f eeds f acid, ml./l. I0 05 1.0

ensities are nee e ,e.g., a eas a. ,an or sp 0 Hydmflumic acid 800 f.p.m. or greater, current densities of at least 600-700 (48%),mL/l. 1.4 2.0 a./ft. are required, and that even higher current densities, PF -P 220 320 220 e.g., 800 a./ft. or more are required for best plating efficieni f ""F'" density [11 plating cles passcs,a./ft. 700 700 700 Products of the above-described characteristics can be Treating time, r,. in produced by the practice of the inventive process, wherein the Plating passes- Cathodic current process conditions may be summarized as follows. d

ensity in oxide pass, a./ft. 200 212 212 Process Parameter Value Solution temperature, F. 106 I09 H7 1. Elcctrolyie a. chromic acid, g./l. broadly, The products produced by the use of the Table 3 processes, including weights of metallic chromium layers and hydrated preferably, m 60 chromium oxide films produced, together with performance b. catalyst of those products when tested for can fabrication property (I )sulfalc caral s akm 35 requirements in accordance with certain test procedures (as hereinafter described), are given in Table 4 below.

TABLE 4 Cr, mg./ft. Ave. peel Stack Humid strength, Enamel Citric acid rust storage Appearance after Ave. Ave. lbs. process test, line test; test lacquering and Example metal oxide in. adhesion width, mm. (60 days) (30 days) baking 9.8 0.6 81 0.1 0 0 No darkening. 5.8 0.7 77 1 0.1 0 1 Do. 6.8 0.7 68 0 0.1 0 1 D0.

1 Oxide determined as mg. Cr+++/fi:. of article surface. 0=no enamel removed; l0=complete enamel removal.

3 0=no rusting; 10=complete rusting.

(2) mixed catalyst in addition to the above operating conditions, the steel article to be treated, which may be, for example, cold rolled, singleor double-reduced carbon steel, e.g., of to 90 pounds basis weight (weight per base box), is preferably cleaned by a usual electrolytic alkaline treatment, spray rinsed, and electrolytically pickled, e.g., in 4 percent sulfuric acid solution, and again rinsed, before being subjected to the process of the invention.

After application of the inventive treatment, the product is usefully water rinsed, hot air dried, and electrostatically oiled before coiling or shearing.

It will be seen from the Table 4 data, that both a metallic chromium layer and an overlying hydrated chromium oxide film were produced in the case of each of these three exemplary processes of the invention, the metallic chromium layer comprising from 5.8 to 9.8 mg./ft. of chromium, and the oxide film containing from 0.6 to 0.7 mg. Cr-l-l-Hft. of article surface.

The products of the aforesaid examples of the inventive process were tested to determine their conformity to the necessary product characteristics, as above-described, for food and beverage container use, the test procedures being as follows, with results as given in Table 4 above.

ADHESIVE PEEL STRENGTH TEST This test is used to measure the effect of the steel stock surface of the strength of a cemented, lap-jointed side seam in tin-free steel cans.

in accordance with this test, flat panel samples (4X6 inches) of the treated steel stock were dip-coated with a lacquer to be used as the protective coating on the fabricated can and for which the adhesion to the can stock was to be determined.

Such lacquered panels were air dried for ID minutes at room temperature and then oven-cured at 415 F. for 30 minutes.

The panels were then cut into three-quarter inch wide strips. Three nylon 11 pellets were heat-tacked to one of the lacquered strips for each of the treated steels to be tested, near one end thereof and equally spaced across the width of the strip. Another strip was positioned over the first and the two were wrapped in aluminum foil, clamped between heated (500 F.) platens (shimmed apart to maintain a 3 mil adhesive layer between the bonded test strips), the pressure raised to 8,000 p.s.i.g. and held at that value for .3 seconds. The specimens were then removed and allowed to cool at room temperature.

The strength of the resulting adhesive bond was determined by means of a standard tensile testing device provided with a special 180 peeling mandrel and operated at a speed of 2 inches per minute.

As shown by Table 4 data, such tests of the products of Examples l-3, using several proprietary can-making quality adhesives, showed the samples to have peel strengths of from 68 to 71 pounds per three-quarter inch of sample widthfar above the minimum pounds per inch requirement for steels of such thicknesses.

ACCELERATED SALT WATER (ENAMEL PROCESS ADHESION) TEST This test is used to measure the degree of adhesion of can coating lacquers and enamels to the steel stock. Panel specimens, 4X6 inches, were provided with a dip-coating of a gold phenolic lacquer (No. 1457 Gold Lacquer, manufactured by lnterchemical Corporation, of Clifton, N. J thinned to give a coating weight, after baking, of 1.8-2.2 mg./in. of panel surface area.

The panels were air dried for 10 minutes and then oven cured for 10 minutes at 410 F.

The panels were then placed in an aqueous solution consisting of sodium chloride, g./l., and 30 percent hydrogen peroxide, l0 ml./l., and maintained therein, at a temperature of IS 0 F for l and 2 hours. Thesamples were then removed, rinsed in cold water, blotted dry and immediately subjected to the Scotch Tape Test, wherein pressure-sensitive adhesive tape is applied to the sample and then quickly and forcibly removed.

As will be seen from the Table 4 data, the samples thereof, when subjected to this test, showed no loss or only a very slight loss of lacquer.

CITRIC ACID TEST This test is used to measure the resistance of the treated steel stock to undercutting of an overlying can-coating lacquer or enamel by corrosive can contents, and simulates undercutting conditions encountered at the site of scratches, breaks or holes in the lacquer on carbonated beverage cans.

Panels were prepared and baked in the same manner as for to F. The specimens were then removed, blotted dry,

and the total width of the cut lines was measured with a 7 power magnifying glass having an 0. l mm. scale on its field.

Increase in scratched line width to more than 0.2 mm. is

considered as excessively severe undercutting.

From the Table 4 data, it will be seen that in no instance did the width of the specimen lines increase measurably.

STACK RUST RESISTANCE TEST Acceptable can stock should show no rusting after one months storage at a relative humidity of percent at 85 F. The present test is more severe and is thus a strict measure of the usefulness of a surface treatment to inhibit rusting during shipment and storage prior to lacquer application.

In accordance with this test, 4X7-inch panel samples of the treated steels were placed in a tight stack, and the stack placed in a scaled dessicator held, at ambient room temperature and percent relative humidity for 60 days.

As shown in Table 4, no rusting, even under these severe conditions, were observed in the case of any of the three exemplary products.

HUMID STORAGE RUST RESISTANCE TEST This is a more severe rusting test than the Stack Rust Test, and represents conditions that would rarely, if ever, be encountered under actual service conditions. In accordance with this test, unlacquered panels of the treated steels, in the form of 4 6-inch panels, were placed, at an angle of 15, in slotted plastic racks with %-inch separations between panels. The racks were placed in a sealed humidity cabinet at l00 F. and, 85 percent relative humidity for 30 days.

Inspection of the data of Table 4 shows that no rusting was observed in the case of the product of Example 1, and that there was only very slight rusting of the products of the processes of Examples 2 and 3.

For appearance determination, the samples were coated with a clear epoxy phenolic enamel, air dried for 10 minutes, then baked for 15 minutes at 415 F. The enamel film weight after baking was 1.8 to 2.2 mg./in. of sample surface. The

panels were then graded for degree of discoloration.

As is evident from the Table 4 data, none of the exemplary products, having oxide weights of 0.6 to 0.7 mg. Cr-ll--l/ft. showed any discoloration.

The processes of Examples l-3 of Tables 3 and 4 were conducted, and the products thereof produced, on a continuous pilot line basis. Additional, commercial-scale runs were also made, using process conditions .within and without the inventive ranges above specified. These additional tests further illustrate the necessary and critical process parameter limits of the invention. In each case, the preplating treatments, as above-described, were applied to the steel article before subjection to the coating process of the invention.

Thus, Table 5 sets forth a series of 32 tests (8 coils) conducted on a modified commercial tin-plate line, wherein two vertical 5-foot deep plating tanks were used, each consisting of a first down leg or pass and a second "up leg or pass, separated by a vertically extending divider or baffle. The tanks were internally coated with polyvinyl chloride, and each was provided with protectively covered sink rolls and hold down rolls. Electrodes were of one inch thick leadl percent silver alloy. The effective length of each electrode in the first three plating" passes (Zone 1) was 23 inches, and the effective length of the electrode in the second zone of the oxide" pass (Zone 2) was 5 feet.

As will be seen from Table 5, the line speed was varied from 380 to 860 f.p.m., Zone 1 current density was initially a nominal 500 a./ft. (actually 504-540 a./ft. and later raised to 630 a./ft. (the maximum available with the electrode configuration used and the existing electrical equipment). For each coil, Zone 2 (oxide pass) current density was varied from O to 200 a./ft.

The products of test Nos. 2A and 28, wherein the line speed was 850 f.p.m. and the Zone 1 current density was only 540 a./ft. showed an incomplete and thin (average under 1.0 mg./ft. chromium layer, and thin (average 0.2-0.3 mg. CrH-l-lft?) oxide film.

Further, test Nos. 4A-4E, run at a line speed of 840 f.p.m. and a Zone 1 current density of 630 a./tt. although resulting in products having a continuous and substantial chromium layer and an excellent oxide film, showed a poor plating efficiency (about 8l0 percent) and a relatively low chromium layer weight. Clearly, higher Zone 1 current densities are required for best, most economical operation of the inventive process when line speeds upwardly of 800 f.p.m. are used.

The results of property tests, as above described, on the Table 5 products are given in Table 6.

3 14 TABLE 6 From Table 6, it will be seen that, with the exception of the Avg peel 2A and 213 products and, to a lesser extent, the 4A-E rue-litvgJ 1 s tr ng h. I Enam l C lt i wid products, the Table products gave good results under each Metal Oxide 'j flff 53353 5 2 fi i 2 2 of the test conditions to which they were subjected.

Products of 2A and 28, with a thin, discontinuous chromi- B T B T B T B T B um layer, and a hydrated chromium oxide containing only 0.2 .6 .3 55 2% 8 to 0.3 mg. Cr-l-t-i-lft. are seen to have been entirely un- 3! 1g :3 53 i satisfactory as regards enamel adhesion and undercutting by 5 g7 59 52 g i 8-} 8-% the citric acid test. Clearly a thicker, continuous chromium 'g :2 :2 2g 28 0 0 1() coating is required, together with a hydrated chromium oxide 63 6.4 .5 .6 5g 22 0 i film heavier than 0.3 mg. CrHl-/ft. 5: f1? :2 2 g 2 5 Products of 4A-4E on the strip bottom, where the chromi- 2.2 1.3 .8 .9 45 22 1 g 8.1 1.3 um layer fell as low-as 1.3 mg./ft. showed relatively poor 3:? 1'2 I; :3 2g 49 1 5 6 results on both the enamel adhesion and a citric acid tests. 6.8 gt] .7 .6 2? 21 0 g 8-} 8-1 Others of the Table 5 products showed good results on the 2:; :2 I; I; 50 48 8 0 1 Table 6 tests, and it is clearly seen from the Table 6 data that 6.3 1 5.6 .7 .7 22 2 9 8 8 8% 8% chromium layer of at least about 1.5 mg./ft. is required 5:8 I; 55 51 0 0 together with an hydrated chromium oxide film containing at .8 -8 -8 7 2 g 3 3% 8-1 least about 0.5 mg. Cr-t-H-lftl 1 2: 3:2 :2 :3 2g g 0 0 It is further notable from the Table 6 results that lower peel 1 1- 5 50 42 g 8% H strengths were generally obtained when higher Zone 2 current big 31% :2 j 0 0 densities were used, and that the lowest measured peel 10.; 9. 5 .6 .7 ,8 3 g-i strength (42 pounds for product 6E) was observed with a high 1 8:: :2 0 0 weight of oxide film (1.5 mg. Cr+-||-/ft. )-again indicating 7.9 7.: .Z 3 g g 8 3-1 that excessively heavy oxide films (e.g., over about 1.5 mg. H :2 0 0 Cr-H-Hft?) are to be avoided for this additional reason. 3.2 7.; .6 8 g 8 8 The effect of line speed and Zone 1 current density (at a constant Zone 2 current density of 100 a./ft. was explored in 1Oxide determlnated 85 CF17 P Square foot of article Surface a further series of tests, as given in Table 7, and wherein the igifig flgg ii fi gigtgi enamel removed plating passes (Zone 1) were provided with shorter electrodes t y grflde- L38 feet length) to enable the application to the moving strip E .2f W.-.- 2 of hi her Zone 1 current densities.

TABLE 7 Chromium, mgJIt. Current Metal Oxide density Time, Line amps/it. sec. Coulon'tbeltt. Top Bottom Top Bottom Plating Appearance Coil speed, eflieiency, No. f.p.m. Zonel Zone2 T1 T2 Zonel Zone2 Total E C E O E E 0 .percent Top Bottom 490 800 100 .508 612 406 61 467 8.2 7.8 9.0 9.9 .5 .6 21.3 27.0 Colorless" Colorless. 690 800 100 .360 434 288 43 331 5.6 5.4 5.7 6.0 .5 .6 20.8 23.2 d0 Do. 1,000 800 100 .249 300 199 30 229 3.9 3.2 3.4 3.4 .6 .6 17.9 19.0 D0. 0 994 100 .488 588 485 59 544 10.6 11.9 11.7 11.6 .8 .9 27.2 26.5 D0. 800 994 100 .311 385 309 39 348 6.3 6.4 6.5 6.6 .8 .8 22.9 23.6 D0. 1,000 994 100 .249 300 248 30 278 4.8 4.7 5.5 5.2 1.0 .9 21.1 23.3 Do. 0 800 100 .498 600 398 60 458 7.6 7.1 7.8 8.1 .4 .5 19.8 22.6 Do. 790 800 .315 380 252 38 290 4.7 4.6 4.8 4.6 .5 .5 20.2 20.2 Do. 1,000 800 100 .249 300 199 30 229 3.5 3.1 3.7 3.6 .6 .5 17.3 20.1 Do. 0 994 100 .508 613 505 61 566 10.3 9.9 10.6 10.4 .7 6 21 8 22.8 Do. 0 994 100 .307 371 305 37 342 1,000 994 109 .249 300 247 30 277 4.7 4.7 4.9 4.9 .8 .7 2 22.0 Colorless" Do. 0 875 100 .498 600 435 60 495 9.3 8.7 9.5 0.2 .7 .7 2 23.5 do Do. 800 875 100 .311 375 272 38 310 5.3 5.2 5.5 5.5 .5 .6 2 22.4 D0. 1,000 875 100 .249 300 218 30 248 4.3 4.0 4.5 4.3 .6 .6 4 21.9 Do. 500 100 .498 600 249 60 309 3.6 3.4 3.8 4.0 .5 .5 2 17.8 Do. 800 500 100 .311 375 156 as 194 0 0 1.0 2.2 .3 .3 0 15.7 Do. 1 Length of Zone 1=1.38 it. per e1ectrodeX3 electr0des=4.15 it.; resi- Length of Zone 2=5 ft. (single electrode); residence time of strip in Zone dence t me of strip in Zone 1=4.15 ft. +line speed (f.p.rn.) X60 see/min. 2:5 it.+line speedXfiO. P ft.heeqe ri 399 9-.

TABLE 8 Avg. peel Enamel process Cr. mg./it. 1 (avg) strength, adhesion (1 hour) 2 Citric acid test, lbs. per line width, mm. Metal Oxide inch Top Bottom M top and M Top Bottom Top Bottom Top Bottom bottom E C E E C E (avg) (avg) 8.0 9.5 .6 .6 49 1 1 1 1 0 0 0.1 0.1 5.5 5.9 .5 .6 52 l 1 0 1 1 2 0.1 0.1 3.6 3.4 .6 .6 51 0 0 1 0 1 1 0.1 0.1 11.6 11.7 .6 .8 53 1 0 o 2 0 0 0.1 0.1 6.4 6.6 .7 .7 54 1 0 0 1 1 1 0.1 0.1 4.8 6.4 .6 .8 50 1 0 0 0 1 1 0.1 0.1 7.4 8.0 .4 .5 52 0 0 0 0 0 1 0.1 0.1 4.7 4.7 .5 .5 50 0 1 0 0 1 2 0.1 0.1 3.3 3.7 .5 .5 53 0 0 0 o 1 2 0.1 0.1 10.1 10.5 .7 .6 50 0 1 0 1 0 0 0.1 0.1 47 6 ":7 51 0 0 0 1 0 (i 01 0.1 9.0 7 .7 42 0 0 0 0 0 0 0.1 0.1 5.3 5 .6 42 0 0 o 0 0 0 0.1 0.1 4.2 6 .6 46 0 0 1 0 0 0 0.1 0.1 3.5 5 .5 44 0 0 0 0 0 0 0.1 0.1 0 3 .3 24 6 10 7 6 10 5 3.9 2.5

1 Oxide determined as mg. Cr+++/ft.

Z 0=No enamel removed; l0=a1l enamel removed. 0=no rust; 10=heavy t.

4 -day grade.

5 day grade from the 'l'ablc 7 data it is seenthat, with the use ofa 100 am. Zone 2 current density, good products, in terms of adequate weight of chromium layer and colorless chromium oxide film of substantial weight are obtainable over a wide line speed range of 500 to 1000 f.p.m. except where the Zone 1 current density of 500 a./ft. is applied to the steel over shortest periods of time, as those corresponding to a line speed of about 800 f.p.m. In the latter instance (test SE), a discontinuous, thin (1.0 mg./ft. or less) chromium layer was obtained, together with 0.3 mg. Cr-l-l-l-lft. in the oxide film.

Table 8, setting forth the results of property tests on the Table 7 products, shows that those products gav good results except, again, for the 55 product which gave unacceptable results under all of the test conditions, i.e., low (24 pounds) peel strength, low enamel adhesion and high undercutting corrosion in the critic acid test.

A still further series of commercial-scale tests is set forth in Table 9, wherein line speeds from 300 to almost 1000 (980) f.p.m. were used, in conjunction with a number of different Zone 1 and Zone 2 current densities.

The Table 9 data confirm the capability of the inventive process to produce, at a high plating efficiency, continuous layer, protective chromium deposits, eg, of 3.5 or 4 to 8 or 9 mg./ft. together with an hydrated chromium oxide film in the range of 0.5 to 1.5 mg. CH-l-HftF, throughout the latter line speed range and at Zone 1 current densities of 500 a./ft. or higher, and a Zone 2 current density ofabout 100 a./ft.

In test series 6A-6F of Table 9, variable Zone 2 current densities from to 795 a./ft. were applied to one side of the strip, and it is seen from the Table 9 data thereon, that Zone 2 current densities of 298 a./ft. and higher resulted in heavy oxide deposits (over 1.5 mg. Cr-l-l-l-lft?) with a resultant discoloration of the product.

Coil 13, comprising test Nos. 13H (coil head), 13C (coil center) and 13T (coil tail), showed heavy (over 2 mg. Cr-l-H lft?) oxide deposits on the slow-down" portions of the coil 13H and 13C), caused by very high current densities (at least about 800 a./ft. at low line speed (about 300 f.p.m.), with a consequent streaking of the product.

Table gives the results of property determinations of the Table 9 products.

The generally excellent protection afforded by the inventive treatment to the Table 9 products is readily observable in the Table 10 data.

Table l 1 sets forth certain yet further large scale tests, series P1-Pl2, wherein, at line speeds of 500 and 800 f.p.m., and Zone 1 current densities of 500 and 800 a./ft. respectively, the Zone 2 current density was varied from 0 to 100 a./ft. and, in 25 a./ft. increments, between 100 and 200 a./ft. Table l 1 also sets forth a further series, 2A-2E-2I wherein an electrolyte was used containing 61.7 g./l. CrO together with 0.79 g./l. of sulfate ion and 0.69 g./l. of fluoride ion (all of the preceding commercial-scale tests having been conducted with a similar electrolyte containing about 50 g./l. of CrO and about 0.9 to 1.0 g./l. ofsulfate ion.

As will be readily seen from the Table l 1 data. Zone 2 current densities under 125 a./ft. resulted, in both the Pl-Pb (500 f.p.m., 800 a./ft. tests, in a colorless hydrated chromium oxide film, whereas higher current densities gave products with at least some coloration.

The Pl-P6 series especially clearly shows the increasing oxide weight, from 0.6 or 0.7 mg. CH++-/ft. to 1.5 or 2.0 mg. Cr-l-H-IftF, with increase in Zone 2 current density from 0 to 200 a./ft.

As shown by the 2A2E-2l series of Table 11, an electrolyte CrO concentration of 60 g./l. is equally effective as the lower 50 g./l. used in the earlier tests in producing the required chromium and hydrated chromium oxide coatings. Peel tests on the product of Test 2A showed values from 52 to 70 pounds per three-quarter inch and critic acid line widths of 0.1 mm. The 2A product showed no visible rusting after 28 days exposure in the Stack Rust Test and no rusting, to grade 1 or 2 rusting (very slight), on 14 days exposure to the Humid Storage Test.

In the case of each of the above-described large-scale tests, the electrolyte temperature was in the aforesaid range of '-l 20bL F., generally under l10-1 12 F.

It will be seen from the foregoing tests and examples that the process of the invention provides a novel and improved process for the manufacture of chromium-plated steel stock for the fabrication of cans and other finished articles requiring a bright, metallic finish together with enhanced oxidationand corrosion-resistance, as well as excellent adhesion to the enamels, lacquers and the like used in the production of such finished goods.

While the prior art teaches steel base articles having thin electrodeposited chromium coatings with an without overlying chromium oxide films, the prior art has not, to out knowledge, evidenced awareness of or the practice of the production of such articles wherein the critical limitations of chromium and chromium oxide thickness have been recognized and produced to provide stock material which will uniformly and consistently meet the demanding property requirements imposed by the users of such materials.

Nor has it heretofore been possible to obtain products of the general character of the specific products of this invention in so simple, effective and low-cost a manner as that herein provided. The essential steps of this invention are easily and quickly manipulatable to provide a range of products, for example, those having varying chromium layer weights, while retaining the limited amounts of overlying chromium oxide essential to proper product performance. The process is easily adapted to base metals having different affinities for electrodeposited chromium, to thereby produce products of uniform characteristics as regards end-use requirements.

It is to be understood that the foregoing examples and specific embodiments are merely illustrative of the principles of the invention, and that various modifications and additions may be made thereto by those skilled in the art without departing from the spirit and scope of the invention.

- TABLE 10 Cr, mg./t't. (avg) l Humid storuuo "M Enamel process Citric acid test Stock rust test test Metal Oxide adhesion 2 line width mm. (30 days) (26 days) Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom 13. 6 l7. 1 .6 1 0. 1 0.1 0 0 8 4 13.5 16.7 .6 .6 2 0 0.1 0.1 0 0 8 6.6 9.4 .5 .7 2 1 0.1 0.1 0 0 9 5 13. 2 16.7 .7 7 1 0 0.1 0.1 0 0 0 4 4.7 7.0 .7 .7 l 1 0.1 0.1 0 0 9 6 4. 9 5. 4 6 6 2 2 0. 1 0. 1 0 0 7 3 4.6 4. 6 .6 .6 1 0 0. 1 0.1 0 0 3 2 4. 5 4. 5 6 6 0 0 0. 1 0.1 0 0 7 2 4.6 4.6 .6 .6 1 0 0.1 0.1 0 0 4 l 4. 5 4.8 7 .6 O 0 0. 1 0.1 (l 0 3 l 4. 2 4. 9 .6 .7 0 1 0.1 0.1 0 0 4 2 4.6 4.9 .6 .6 1 1 0.1 0.1 0 O 4 2 4. 7 4.9 6 .6 0 0 0.1 0. 1 0 0 2 4 4. 5 5.0 .4 5 1 l 0.1 0. 1 0 0 l 3 4. 5 4. 6 6 .6 0 1 0. 1 0. 1 0 0 l 2 4. 5 4. 9 5 .6 1 0 0. 1 0. 1 O 0 l 2 4.4 4. 8 .6 .6 2 2 0. 1 0. 1 0 0 1 2 4.5 4.7 .6 .7 2 1 0.1 0.1 0 0 1 6 6.2 7.3 .7 .6 1 0 0.1 0.1 0 0 3 1 4. 2 4. 5 7 6 2 1 0. 1 0. 1 0 0 1 0 4. 3 4. 6 .6 .5 1 0 0. 1 0.1 0 0 0 0 4.0 4. 5 7 6 0 0 0. 1 0. 1 0 0 1 1 4.0 3.8 .6 .8 1 0 0.1 0.2 0 0 3 3 3.6 4.1 .9 1.0 0 0 0.1 0.1 0 0 5 4 4.4 4. 9 8 .7 0 0 0. 1 0.1 0 0 2 1 4. 2 4. 3 8 8 2 0 0. 1 0. 1 0 0 3 2 3.6 4.5 1.2 1.1 1 0 0.1 0.1 0 0 8 5 3.5 4.1 1.1 1.1 1 0 0.2 0.4 0 0 9 4 3.8 3.8 1.0 1.1 0 0 0.2 0.2 0 0 6 4 3.5 4.3 .9 1.0 1 0 0.2 0.2 0 0 6 4 3.9 4. 5 8 .9 0 0 1. 1 0. 2 0 0 3 2 3.4 4.4 9 .9 0 0 0.1 0. 1 0 0 3 1 3.4 4.1 1.0 .7 0 0 0.1 0.1 0 0 3 l 3.5 4.3 1. 1 .7 0 0 6.1 0. 1 0 0 1 1 3.6 4.6 1.0 .7 0 0 0.1 0.1 0 0 3 1 4.4 4.4 1.0 .8 0 0 0.1 0.1 0 0 5 2 4. 3 4.8 .8 .7 1 0 0. 1 0. 1 0 0 5 2 16. 6 17.4 2.2 2. 0 0 0 0. 1 0.1 0 0 1 0 3.8 5.0 5 .7 0 0 0. 1 0. 1 0 0 1 1 17.0 17.5 2.6 2.3 3 1 0. 1 0. 1 0 0 5 1 4. 6 4. 8 7 6 0 0 0. 1 0. 1 0 0 2 1 5.0 5.2 .7 .6 0 0 0. 1 0. 1 0 0 3 1 7. 6 8.4 .6 8 1 0 0. 1 0. 1 0 0 3 2 4.6 4.6 .7 .7 1 0 0. 1 0. 1 0 0 3 2 8.2 9. 5 6 6 1 0 0. 1 0. 1 0 0 2 1 5.1 4.0 1.5 .7 2 0 0.1 0.1 0 0 5 2 4.8 4.4 1.6 .7 0 1 0.1 0.1 0 0 4 2 4.7 4.4 1.4 .7 1 1 0.1 0.1 0 0 5 3 4. 4 4. 4 8 7 1 1 0. 1 0. 1 0 0 6 2 4.4 4.4 6 .7 2 0 0.1 0. 1 0 0 7 3 4.4 4.4 .7 .9 2 1 0. 1 0. 1 0 0 5 4 l Oxide was determined as mg. Cr -Vftfi. Z O=No enamel removed: 10=al1 enamel removed. 3 0=No rust; 10=heavy rust.

We claim: 1. A continuous process for the production of bright, chromium plated, rolled steel stock comprising in a first stage, plating a steel product as cathode with duplex coating made up of a metallic Cr layer and an overlying chromium containing oxide layer in which the metallic layer is thicker than 1.5 mg./ft.'* and the oxide layer is greater than 1 mg./ft. in thickness, passing said cathode to a second stage in which said stock is maintained in an aqueous electrolyte consisting essentially of 40-100 g./l. CrO and a catalyst selected from the group consisting of a. 0.2 to 3.6 g./l. sulfate, or b. 0.2 to 3.6 g./l. sulfate and 0.1 to 2.0 g./l. fluoride for a time sufficient to dissolve at least 0.5 mg./ft. of oxide therefrom, and to provide an oxide with a thickness of 0.3 to 2.0 mg./ft.

2. A process in accordance with claim 1, in which said dissolution time is sufficient to provide an oxide with a thickness of0.3 to 1.5 mg./ft.

3. An article produced by the process of claim 2, wherein the metallic chromium layer has a maximum thickness of about 12 mg./ft. of article surface.

4. A process in accordance with claim 2, in which subsequent to said second stage, the cathode is passed to,

a third stage for cfiecting further control of the nature and thickness of said oxide by applying to said stock a cathodic current below about 500 a./ft. through an aqueous electrolyte consisting essentially of 40400 g./l. CrO and a catalyst selected from the group consisting of a. 0.2 to 3.6 g./l. sulfate, or b. 0.2 to 3.6 g./l. sulfate and 0.1 to 2.0

wherein the cathodic .concentration of all three stages is substantially the same.

10. A process in accord with claim 9, wherein said cathode is passed at a line speed greater than 800 ft./min. and the cathodic current in said first stage is at least about 600 a./ft.

11. A process in accord with claim 9, wherein the operations of all three stages are accomplished in the same electrolyte bath.

- Patent No. 3,642, 587 Dated February 15, 1972 a T i Inventor(s) William S Allen, et 8.1

It is certified that error appears in the above-identified patent and that said Letters Patentare hereby corrected as shown below:

Column 4, line 71,, "would" should be could Columns. 19 and Z0,- the tables following Table 9 Continued, the tables should be identified as Table 11 and Table 11 Continued, Column 21, line 51, "product" should be StOCK line 65, "accordance" should be accord line 71, "accordance" should be accord Signed and sealed this 8th day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PC4050 (10-59) USCOMM-DC 6O376-P69 i .5. GOVERNMENT PRINTING OFFICE: I959 0-366-33L 

2. A process in accordance with claim 1, in which said dissolution time is sufficient to provide an oxide with a thickness of 0.3 to 1.5 mg./ft.2.
 3. An article produced by the process of claim 2, wherein the metallic chromium layer has a maximum thickness of about 12 mg./ft.2 of article surface.
 4. A process in accordance with claim 2, in which subsequent to said second stage, the cathode is passed to, a third stage for effecting further control of the nature and thickness of said oxide by applying to said stock a cathodic current below about 500 a./ft.2, through an aqueous elEctrolyte consisting essentially of 40-100 g./l. CrO3 and a catalyst selected from the group consisting of a. 0.2 to 3.6 g./l. sulfate, or b. 0.2 to 3.6 g./l. sulfate and 0.1 to 2.0 g./l. fluoride.
 5. A process in accord with claim 4, wherein said cathodic application is applied for a time sufficient to provide an oxide thickness of at least 0.5 mg./ft.2.
 6. A process in accord with claim 5, wherein the cathodic current density of stage 3 is below about 125 a./ft.2.
 7. An article produced by the process of claim 6, wherein the metallic chromium layer has a maximum thickness of about 12 mg./ft.2 of article surface.
 8. A process in accord with claim 6, in which the electrolyte concentration of stage 2 is substantially the same as stage
 3. 9. A process in accord with claim 8, in which the electrolyte concentration of all three stages is substantially the same.
 10. A process in accord with claim 9, wherein said cathode is passed at a line speed greater than 800 ft./min. and the cathodic current in said first stage is at least about 600 a./ft.2.
 11. A process in accord with claim 9, wherein the operations of all three stages are accomplished in the same electrolyte bath. 